This invention relates to an image data transmission system for transmitting an image data signal from a craft, such as a spacecraft, to an earth station in order to obtain a stereo image of an object zone which is typically on the ground surface. Such an image data transmission system is particularly useful in a remote sensing system for a planet, such as the earth.
An image data transmission system of the type described is used to remotely sense an object zone on board a craft flying over the object zone along a flying direction at a predetermined flight altitude. In a remote sensing system, it is often necessary to pick up a stereo image, such as a stereo topographic image, from the object zone by the use of an image pickup apparatus. In order to provide the stereo image, the object zone should be picked up, along the flying direction at two positions spaced apart from each other, so as to form two optical images of the object zone. As will become clear, the craft carries the image pickup apparatus comprising an optical system for producing first and second partial optical images of first and second partial zones of the object zone, which are spaced apart from each other. The image data transmission system comprises first and second photoelectric transducers for transducing the first and the second partial optical images to first and second partial electric signals, respectively. The first and the second partial electric signals are transmitted, as an image data signal, by a transmitter from the craft to an earth station.
A recent requirement is to provide the stereo topographic or a like image at a high resolution so as to make up a precise stereo topography or the like. This requirement gives rise to an increase of data quantity of the image data signal. However, the craft has a restricted data transmission rate. This means that it is necessary to carry out data compression of the first and the second partial electric signals.
A conventional image data transmission system by the use of the data compression is disclosed in U.S. Pat. No. 5,134,473 issued to Riichi Nagura, namely, the instant applicant, and assigned to the instant assignee. In brief, the image data transmission system comprises the first photoelectric transducer for producing the first partial electric signal representative of the first partial optical image of the first partial zone lying forwardly of the craft flying at a first position and the second photoelectric transducer for producing the second partial electric signal representative of the second partial optical image of the second partial zone lying directly under the craft flying, an interval of time later, at a second position spaced apart from the first position by a distance between the first and the second partial zones picked up by the first and the second photoelectric transducers when the craft is at the first position. The image data transmission system further comprises a compression unit for compressing the first and the second partial electric signals into first and second compressed signals. A delay circuits gives the first compressed signal a delay equal to the interval of time and produces a delayed signal. A subtracter calculates a difference between the delayed signal and the second compressed signal and produces a difference signal. The first compressed signal and the difference signal are transmitted by a transmitter, as the image data signal, to the earth station to obtain the stereo image of the object zone.
In the data transmission system described above, it is inevitable that the stereo image has restricted quality.